The present invention relates to a photoreceptor and more particularly to, e.g., an electrophotographic photoreceptor.
There are conventionally known those electrophotographic photo-receptors such as selenium photoreceptors, As-, Te- or Sb-doped selenium photoreceptors, ZnO- or CdS-dispersed resin binder-having photoreceptors, and the like. However, these photoreceptors have problems with respect to environmental pollution, the thermal stability and the mechanical strength thereof. On the other hand, in recent years there have been proposed electrophotographic photo-receptors comprised principally of amorphous silicon (a-Si). The a-Si has the so-called dangling bond which is formed by the severing of the bonding of Si-Si, and this defect causes many localized levels to be present inside the energy gap. For this reason, the hopping conduction of the thermal excitation carrier is such as to cause the dark resistance to be small, and the photo-excitation carrier is trapped by the localized levels to deteriorate the photoconductivity. Accordingly, the above-mentioned defect is compensated by a hydrogen atom(H) to bond the H to Si to thereby fill the gap of the dangling bond.
The resistivity in the dark of this amorphous hydrogenated silicon (hereinafter referred to as a-Si:H) is from 10.sup.8 to 10.sup.9 .OMEGA.-cm which is as small as about 1/10,000th of that of amorphous Se. Thus, a photoreceptor comprising a single a-Si:H layer has the problem that the dark attenuation speed of the surface potential thereof is high and the initial charging potential is low. On the other hand, however, if the layer is subjected to the irradiation of a visible light or of a light in the infrared region, the resistivity thereof becomes reduced greatly, so that the layer has very excellent characteristics as the photosensitive layer of the photoreceptor.
For providing the a-Si:H with a potential retainability, although the resistivity thereof can be increased up to about 10.sup.12 .OMEGA.-cm by doping boron thereinside, not only is it difficult to control precisely the amount of boron but even the resistivity of about 10.sup.12 .OMEGA.-cm is not sufficient in the charge retainability for use in the photosensitizing process by the Carlson method.
Further, it is possible to obtain as high a resistance as 10.sup.13 .OMEGA.-cm by the introduction of a slight amount of oxygen together with boron, but if this is used in the photoreceptor, the photoconductivity thereof becomes reduced, resulting in a problem that the toe portion of the characteristic curve is deteriorated or residual potential occurs. This results in the problem that the sharp-cutness of the toe portion of the characteristic curve is deteriorated or residual potential occurs.
No thorough study has been made so far on the chemical stability of photoreceptors having the a-Si:H on the surface thereof against the influence of the exposure thereof to air or moisture over a long period, the influence thereupon of chemical compounds formed during corona discharge, and the like. For example, it is known that when a photoreceptor is exposed to air for more than a month, the receptive potential thereof is markedly deteriorated by the moisture in the air. Further, the a-Si:H has poor adherence to such a support material as aluminum, stainless steel, etc., so that it becomes a problem in making practical use of the a-Si:H as an electrophotographic photoreceptor. As a measure to solve this problem it is known that an adhesion layer comprising a silane coupling agent as disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 87154/1980 or such an organic macromolecular compound as a polyimide resin, triazine resin, or the like, as disclosed in Japanese Patent O.P.I. Publication No. 74257/1981 is provided between the a-Si:H layer and the support. In these instances, however, the formation of the adhesion layer and the formation of the a-Si:H layer must be made separately, requiring the use of an additional layer forming machine, so that the production operation is not efficient. In addition, the obtaining of a better quality of the a-Si:H layer requires keeping the base plate (support) at a temperature of normally about 200.degree. C. or higher during the formation of the layer, but the undercoat adhesion layer cannot withstand such a high temperature.